Glass is typically made from fused Silica and may be shaped to form a lens or window. To focus light, glass reflects and refracts light based on its shape, material properties, and an angle of incident light. If the shape of the glass is distorted, for example by external forces, light rays traveling through the glass, or bouncing off the glass surface, may be directed in an undesired direction. This undesired redirection of light can cause problems, especially with optical systems that rely on the stability of optical characteristics.
With advancements in technology, lenses have been utilized in many different environments including, but not limited to, outer space, where the lens must be enclosed in an air-tight body. Temperature changes can cause thermal expansion problems in a lens assembly.
A typical lens or window assembly contains a lens or a window secured to a lid that may be part of an enclosure for housing electronic components, for example, sensitive detectors. The lens or window may have a metallic coating on one surface of the lens and is secured to the lid by soldering. The lens or window may have a different coefficient of thermal expansion (CTE) than that of the lid. A change in temperature will cause the lid to expand or contract faster or slower than the lens or window, which can impart forces on the lens or window. The lens or window is soldered along one surface and, therefore, the imparted forces are not in the plane of the lens. This can generate a bending moment that can cause the lens to bow enough to adversely affect the optical characteristics of the lens. Bifringence is a by-product of the stresses caused by the bending moment imposed on the glass and may result in unacceptable distortion of images through the lens or window.
The lens, frame, and lid may be coupled together using soldering techniques to provide a hermetic seal. Unfortunately, the seal between the lens and the frame may impart detrimental forces to the lens that may change the shape of the lens enough to cause optical problems. The solder joint may also fail due to these external forces, thereby compromising the hermetic seal. Soldering a minimal clearance joint between the lens and the frame results in a very thin layer of solder. While a thin layer of solder exhibits great strength in certain contexts and types of testing, it does not provide for significant radial compliance in the configuration described above. Accordingly, when the lens assembly is subjected to temperature cycling, as is required in the testing of many military components, the solder joint may fail. In addition, the hard solder joint may not fail during testing and may instead fail during use of the lens or window, which may be located miles above the Earth where repairs are expensive to accomplish, if at all possible.
Thus, a heretofore unaddressed need exists in the lens industry to address the aforementioned deficiencies and inadequacies.